ANN ARBOR—While marijuana is an illegal substance under federal law, the U.S. Justice Department is ignoring smoke signals coming from Washington and Colorado, says a University of Michigan professor.

Both states passed ballot initiatives that legalize the possession of certain amounts of marijuana by adults.

Melvyn Levitsky, a professor at U-M's Ford School of Public Policy, argues that in refusing to challenge legalization laws, the feds are ignoring their duties under the supremacy clause of the U.S. Constitution, the Controlled Substances Act and U.S. treaty obligations.

"I'm concerned about the federal government cherry-picking federal law, deciding which federal laws it's going to enforce," he said.

Levitsky cited the example of Arizona's attempt to pass laws on illegal immigrants. The federal government took the state to the Supreme Court, challenged the state law based on the fact that federal law trumps state law, and won the case. However, it has chosen not to challenge state law on marijuana in Colorado and Washington.

Levitsky said the government is obligated to carry out the provisions of the three international drug conventions that, under the U.S. Constitution, are to become the law of the land.

"If we decide that we're going to ignore our treaty obligations, then what does this mean for other treaties—such treaties as the nonproliferation treaty, a variety of treaties having to do with the environment and with human rights?" he said.

"And what does that mean in terms of the image of the United States? I think this is a major concern that has to be met by the federal government and answered by the federal government. It hasn't done so yet."

Levitsky, former U.S. Ambassador to Brazil, served as Assistant Secretary of State for International Narcotics Matters in 1989-93 and had a seat on the Vienna-based International Narcotics Control Board in 2003-12.